The present invention generally relates to systems and methods for micromachining holes in glass and in objects containing glass. An application for which the present invention is particularly suited is the formation of vias and micro vias in glass reinforced dielectric substrates such as those used in printed circuit boards, ball grid array substrates and similar electrical circuits.
It is known to use lasers to micromachine holes such as vias, micro vias, and blind micro vias, in electrical circuits such as printed circuit boards and interconnect chip packages.
Lasers that are suitable for such applications include lasers generating UV laser beams. A conventional method for generating UV laser beams is to pass a pulsed laser beam output from a Q-switched solid state laser, such as Nd:YAG laser, through one or more non-linear crystals. This methodology, called harmonic generation, results in a frequency modified beam. Conventional neodymium based solid state lasers produce laser beams having a wavelength of about 1064 nm. Third harmonic generation of such a beam results in a beam having a wavelength of about 355 nm. Fourth harmonic generation of such a beam results in a beam having a wavelength of about 266 nm.
Beams having a wavelength in the vicinity of 355 nm can efficiently form holes in copper layers of electrical circuit substrates. However, such beams fail to efficiently form holes in glass reinforced dielectric layers of electrical circuit substrates. Beams having a wavelength in the vicinity of 266 nm can efficiently form holes in both copper layers and glass reinforced dielectric layers of electrical circuit substrates. The use of 266 nm beams however is problematic, for example, due to the loss of power resulting from the fourth harmonic generation process, optical requirements of lenses used with such beams, reduced damage thresholds of optical elements and relatively high cost.
The present invention seeks to provide improved systems and methods for forming holes in glass substrates and other substrates containing glass, such as printed circuit board substrates.
A broad aspect of the invention relates to a laser device for outputting UV laser light having a wavelength in the range of about 285 nm to about 333 nm, and more specifically between about 290 nm and 320 nm.
Another broad aspect of the invention relates to a micromachining system for forming holes in substrates containing glass, in which a frequency converted pulsed UV laser beam having a wavelength in the range of 285 nm-333 nm, and more specifically between about 290 nm-about 320 nm is employed.
Another broad aspect of the invention relates to a micromachining system for forming holes in substrates containing glass in which a sub-1000 nm beam output by a pulsed solid state laser, converted into a UV beam, is employed.
Another broad aspect of the invention relates to systems and methods for fabricating electrical circuits, such as printed circuit boards, containing glass. Electrical circuit fabrication includes depositing a layer of a metallic conductor material on a surface of a dielectric substrate containing glass, forming a hole in the layer of metallic conductor material with a UV laser beam in the range of about 285 nm to about 333 nm, and more specifically between about 290 nm and 320 nm, and then at a location of a hole in the layer metallic conductor, forming a hole in the layer of dielectric substrate containing glass with a UV laser beam in the range of about 285 nm-about 333 nm, and more specifically between about 290 nm and 320 nm.
In accordance with an embodiment of the invention, a UV laser beam having a wavelength in the range of between about 285 nm and about 333 nm, and more specifically between about 290 nm and 320 nm is obtained by outputting a sub-1000 nm laser beam from a solid state laser, such as by a neodymium doped or ytterbium doped laser crystal, and then frequency converting the sub-1000 nm laser beam by means of third harmonic generation to produce a laser beam having a wavelength in the range of about 285 nm to about 333 nm, and more specifically between about 290 nm and 320 nm.
In accordance with an embodiment of the invention, a suitable UV laser beam in the range of about 285 nm to about 333 nm, and more specifically between about 290 nm and 320 nm, is obtained by harmonic generation of an initial sub-1000 nm laser beam. In an embodiment of the invention, a sub-1000 nm laser beam is the product of laser transitions in the range of 870 nm and 980 nm. Such laser transitions are produced, for example, by the laser excitation of neodymium ions or ytterbium ions in suitably doped YAG, YLF, YVO4 and YAlO3 crystals. In an embodiment of the invention, harmonic generation is third harmonic generation resulting from the passing of the sub-1000 nm laser beam through at least one non-linear, or other suitable frequency converting, optical medium.